Patients with Systemic Lupus Erythematosus often have circulating autoantibodies reactive with anionic phospholipid (aPL). The presence of these antibodies correlates with a clinical syndrome of arterial and venous thrombosis, recurrent fetal loss due to placental infarction, thrombocytopenia, and symptoms of cerebral ischemia. The pathogenesis of this hypercoagulable state is not known. The goal of this project is to elucidate the molecular basis of the prothrombotic state associated with aPL and to define genetic and other risk factors that determine risk of thrombosis in patients with aPL. Preliminary data has been obtained showing that lg purified from patients with aPL activate vascular endothelial cells (EC). This was associated with a change in the surface phenotype of the EC such that the cells became adhesive for monocytes. Treatment of EC with lg from control patients did not produce this effect, while rabbit serum from an animal immunized with apoH, a protein cofactor that makes up part of the aPL antigen, mimicked the effect. Plans are to characterize the prothrombotic phenotype induced in EC by aPL and rabbit apoH antibodies, including assays for procoagulant, anti- fibrinolytic and adhesive functions. These assays will be immunological, functional, and molecular. In addition the molecular basis of signal transduction by these antibodies will be explored, with emphasis on the role of the apoH cofactor and potential endogenous EC apoH "receptors". These receptors will be identified, characterized and molecularly cloned. We also propose to study the effect of EC adhesion on monocyte activation, since these cells are known to be important regulators of hemostasis. Since our data suggest that activated Ec are opsonized by aPL we will also explore the role of monocyte Fc receptors in contributing to the prothrombotic state. Engagement of these receptors is known to induce monocyte procoagulant activity. Additionally, activated platelets and apoptotic lymphocytes to which aPL also bind will be used as substrates for FcR-driven monocyte procoagulant activity. To bring these studies to a direct clinical level we will study a large number of patients with aPL. Since our data suggest that apoH and FcR mau play an important role in mediating the prothrombotic state associated with aPL, we will study the role of FcR genetic polymorphisms, apoH isoform, and IgG subclass in determining risk of clinical thrombosis. This will involve a multivariate analysis of a large cohort of SLE patients. In parallel, the role of these inherited factors in modulating the in vitro procoagulant state will be studied. The ultimate goal of these studies is to understand the molecular basis of the aPL syndrome and thus to develop novel diagnostic and therapeutic approaches of these patients.